The present invention relates to the process industry. More specifically, the present invention relates to an isolation diaphragm or "diaphragm seal", of the type used to couple a process measurement instrument to a process.
Some types of process measurement instruments, such as pressure transmitters, have a pressure sensor, which is fluidically coupled to an isolation diaphragm by a fill fluid. The isolation diaphragm comprises part of a sub-assembly called a "remote seal" or a "diaphragm seal" and isolates the pressure sensor from corrosive process fluids being sensed. Pressure is transferred from the isolation diaphragm seal to the sensors through the fill fluid, which is substantially incompressible and fills a capillary tube connected between cavities in both the diaphragm seal and the sensor. When the process medium contacts the remote isolation diaphragm, its pressure is conveyed through the fill fluid to the pressure sensor disposed in the transmitter housing.
The diaphragm, also called a "membrane" is typically bonded to a diaphragm seal body, which supports the outer circumference of the active area of the diaphragm and allows the inner portion of the diaphragm to deflect in response to pressure changes in the process fluid. Some diaphragm seals use extended diaphragms that cover a diaphragm support area of the seal body to form a gasket surface. Such extended diaphragms are often found in diaphragm seals designed for corrosive fluids. By making the diaphragm out of a material that can withstand the corrosive process fluid and by expanding the diameter of the diaphragm to form a gasket surface, the seal body can be made out of a more common and less expensive material.
The diaphragm may be attached to the diaphragm support area of the seal body by brazing the two pieces together. However, the brazing process is somewhat expensive to utilize. To avoid the expense of brazing, the art has adopted the use of two concentric welds to bond the expanded diaphragm to the seal body. One of the concentric welds is made at the outer circumference of the diaphragm and the second weld is made at the inner circumference of the seal body's diaphragm support area.
Although using two concentric welds reduces the cost associated with attaching a diaphragm to a seal body as compared with brazing, prior art systems trapped air under the gasket surface of the diaphragm between the two concentric welds. This air makes the final product look flimsy and susceptible to damage. In addition, if the inner weld fails, the space created by the trapped air or the trapped air itself can change the operational characteristics of the sealed chamber. In addition, this air may expand if the seal system is exposed to vacuum, causing erroneous readings and possible damage to the diaphragm.